Abstract

We use the records of the ground motion obtained at near-fault accelerometers to study the space and time evolution of rupture and faulting during the İzmit earthquake. We find that the rupture propagated at the sub-Rayleigh speed of about 3 km/sec on the western and eastern segments of the fault, but that the central segment (İzmit-Sapanca Lake-Sakarya), nearly 50 km long, broke at the supershear speed of about 4.8 km/sec. This value, within the range of uncertainties, is the one theoretically predicted (

\(\sqrt{2}V_{\mathrm{S}}\)
) in fracture dynamics for stable shear crack growth at intersonic speed. We infer an average fault slip of about 2.9 m over a total rupture length of about 150 km, with the largest values (of up to 6 m) occurring in the Gölcük area to the west and in the Sakarya region to the east. The strong-motion data also indicate that the slip diminished gradually to the west beyond the Hersek peninsula over about 30 km, whereas it stopped abruptly at depth at the termination of the eastern (Karadere) segment. The slip duration is between 2 and 4 sec, except in the hypocentral area, which slipped in about 1 sec. The seismic moment inferred is about 2.5 × 1020 N m.

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